FM200 - Contradiction in NFPA 2001?

[sl3656]

In NFPA 2001-2018, Clause 5.3.6 states that:

"Other than the ventilation systems identified in 5.3.6.2, forced-air ventilating systems, including self-contained air recirculation systems, shall be shut down or closed automatically where their continued operation would adversely affect the performance of the fire extinguishing system or result in propagation of the fire".

5.3.6.2 states that:

"Ventilation systems necessary to ensure safety shall not be required to be shut down upon activation of the fire suppression system. An extended agent discharge shall be provided to maintain the design concentration for the required duration of protection."

---

However, several times throughout the standard, especially in the Annex, NFPA talks about continual mixing, wherein the deployed gas within the protected enclosure is assumed to be agitated to a state where the concentration at the floor will decay at the same rate as the concentration near the ceiling. For example, C.2.8.1.5.2 provides an equation for calculating extinguishants that are heavier than air with continual mixing.

How do I reconcile the apparent contradiction where NFPA permits air-moving equipment is left on during the retention period for continual mixing, yet Clause 5.3.6 does not?

 

[UFT12]

One does not contradict really the other. 5.3.6 addresses and recognize 5.3.6.2. It only means in general ventilation systems be switched off. Makes sense, otherwise reaching the design concentration may not be guaranteed.

Exception to this general rule would be 5.3.6.2 where it is not possible to switch off ventilation systems. These may be absolutely necessary to remain on, either from a life safety point of view or very critical for some expensive process equipment. In this case the exhaust air must be know to calculate extra agent quantity for an extra duration.